Our Man in the Field: The Process of the IP Solution, Part V

April 25, 2005
How to choose the correct lenses for your surveillance cameras

As discussed in the last column, resolution is a very important part of the overall image. Regardless of being an IP or an analog image, if the clarity of the image is too little to determine identity of the object, person, area or whatever it is that you are watching, you may as well save your efforts and not install the system. The next important issues surround what lenses you will choose.

Choosing the proper lens for your application is equal to all other portions combined. If you end up with a view that is too wide, the best resolution camera on the market will not give you a clear image of objects that are too far from the camera. Equally, if you choose too much magnification, the scene will be very narrow and you may be required to use more cameras than you intended to cover the application.

The first step to choosing lenses is to understand that it the decision is 100 percent based upon mathematics and therefore not magic or luck.

Starting at the beginning, we will assume that we are working with the defined camera application. You have your viewing position, purpose and required criteria worked out. We will assume that you have your camera style worked out as well -- that is, what type of camera you will need according to the light levels that you have to work with: day/night, color, low light, etc.

I will not assume that you have decided upon a standard camera or a mega-pixel camera yet, but you should at least have an idea as to whether or not you plan to use a fixed or Pan/Tilt (PT). Right off the bat, if you are planning to use a PT, you will most likely be looking to use a zoom lens, so you will calculate two ratios of lens ... the wide and the telephoto. However, with the IP solution, you may be able to replace the PTZ idea with a fixed mega-pixel camera. Again, it is determined by your application and transmission capabilities.

Let's start with the fixed lens. In order to calculate what lens to use, you must have three points or criteria. You must know the format size of the camera, the distance between the camera and the scene to be viewed, the width of the scene that you wish to view or the height of the scene that you wish to view. Without three of the four mentioned criteria, you cannot calculate a lens.

The width or height of the scene will be based upon what you are trying to see. The size of the objects required in your view for identification or viewing will also matter. Based upon general applications, the object of view should take up at least 10 to 20 percent of the overall width of the scene. The smaller the detail of the object, the more of the scene you must have focused on the object. Personal identification of a human should be 25% of the overall scene width as based upon the SWGIT CCTV guidelines (Scientific Working Group on Imaging Technology; recently adopted by the FBI as a standard on CCTV system design).

From here, there are four or five different methods for calculating a lens. You can use a formula:

SCENE WIDTH = (Format (Hz) X Distance) / Focal Length

For this, you need to know certain statistics about the imager in the camera that you are using, i.e. the horizontal or vertical format size of the imager. The second (and easier) method would be to use a field of view calculator. These wheels or slide rules are available, for free, from almost all of the major manufacturers and are very easy to use.

You could also use a cheat sheet. This is a form that list several different sizes of lens down the left column and general distances in feet or meters across the top. The objective is to match the distance with the lens and come to the point where they cross and see the field of view that you would create.

Some folks will recommend that you use a "view finder" to pick your lens. I, however, will not. The view finder is an excellent tool for field of view verification and customer "hands-on" understanding, but it is not a tool that was designed to actually pick a lens for an application. Lastly, there are several different electronic calculators that are available on the internet (from various manufacturer sites). Regardless of the method that you choose, your final lens will still be based upon what you are trying to see, how much detail, what distance and with what format size camera.

In most cases, with a fixed camera I will recommend that a vari-focal lens be used. This is a lens that is considered to be a limited zoom lens. The vari-focal lens leaves the designer and the installer a bit of lateral motion with the actual final view. If you find that you need a bit more magnification, you can zoom it in; if you need a bit more width, just zoom it out.

Since these vari-focial lenses are a limited zoom, the ratio of zoom usually ranges in the 4:1 to 8:1 area. That is, a 4:1 ratio lens would be a 8mm to 32mm lens. The 8mm measurement refers to the wide portion, and then multiplied by four equals 32mm as the telephoto maximum. To choose a proper vari-focal lens, you should choose a range that would place your fixed lens choice as close to the center as possible.

Going back to the actual calculations, one of the most common errors made by designers is that they don't understand a simple philosophy of distance. If you measure your scene to be 30 feet away and you mount your camera 40 feet in the air, your distance from the camera is not actually 30 feet. It is actually 50 feet. This was originally discovered by a dude named Pythagorus a couple of thousand years ago. His self-named theorem says that A-squared plus B-squared equals C-squared, where A is the distance from the scene as measured on the ground, B is the height of the camera and C is the missing leg.

Let's try it quickly. Thirty feet squared is 900. Forty feet squared is 1600. The square root of these two combined (900 + 1,600 = 2,500) is 50 feet. Yes this works with metric measurements as well and is a very important calculation to remember with all cameras that are mounted above eight feet or so, at least if you want to be accurate or close with your lens choice.

Lastly in our lens calculations is the choice between standard analog or IP cameras of different resolutions and mega-pixel cameras. As you increase the resolution of a camera, you can (exponentially) increase the width of the scene and still capture the quality of image that you require. The advantage is that you may be able to do the work of two, three or even four cameras with one. The attached view (see second photo, above) was provided by Paul Bodell of IQinVision and says it all in a single picture. The advantage of the mega-pixel camera is that you can capture several times the amount of area and still have good resolution at all points within the image. The concern with using higher resolution images in our IP solution is, of course bandwidth for transmission or recording. But, it is still a very viable solution to review and become familiar with ... especially in those large open areas of your design, such as parking lots or production floors.

If you wish to have in-depth, plain-language explanations on lenses, choices, compatibility points and much more, then you should buy one of my books. How's that for a plug?

Meanwhile, in my next column, I will discuss the various formats and types of video transmission for the cross-over analog/IP world. It's the next natural step and your options are growing daily.


For the past 31 years, I have been bragging to friends and family about the security industry and it gifts to the general public. So I am now addressing that same industry, personally with a request for help.

In about six weeks, a small Catholic School on the lower west end of Davenport Iowa will be permanently closing its doors. Sadly, this is happening after 100 years of dedicated service to the education of children of all faiths. This is a school where my wife and her brothers, my three sons and a countless number of other folks that are close to me, have attended. This is a school where the teachers and staff have worked for below average wages and given from their hearts for as many as 25 years. Because we are a small parish and an equally small community, our staff was never afforded compensation or retirement benefits. They will be saying goodbye with nothing more in their pockets other than the fond memories and broken hearts that years educating others will create.

Several weeks ago, I helped to found a committee of parents, parishioners and friends of St. Alphonsus School. The committee's sole purpose is to raise money for an employee appreciation fund to help these departing employees. All costs involved with our project, including legal, financial, postage and marketing have been donated by a wide array of professionals in the area. This allows us to use almost 99 percent of all money raised toward our employees.

Since the funds we are trying to raise are beyond the capacity of our local area, each member of the committee has reached out to friends, neighbors, and fellow workers. I am reaching out to the security industry as a whole. I realize that the world is full of good causes and sad cases. I also realize that this cause does not warrant national attention, and yet here I am. I am not trying to save the world or prevent any major disasters. I am only reaching out to an industry that has given me so much. I am reaching out to you, as an individual or company, to ask you to reach into your heart and help us to say thank you to a small group of people, a group of folks, that just like you, have given so much from their hearts for so long. Please don't think that any donation is too small or to large. All donations are fully tax deductible for those that itemize their taxes. Unfortunately our time is very short as we must close our efforts as of June 1, 2005. I personally have pledged to match 1 percent of all funds raised up to the first $200,000.

Please help me in this worthy cause by sending a check or money order today. These should be made out to: St. Alphonsus Employee Appreciation Fund and sent to the same name at P.O. Box 163, Davenport, 52805.

Thank you, from my heart for whatever you are able to do to help us.

- signed, Charlie R. Pierce

About the Author: Richard R. "Charlie" Pierce has been an active member of the security industry since 1974. He is the founder and past president of LRC Electronics Company, a full service warranty/non-warranty repair center for CCTV equipment. In 1985, Charlie founded LeapFrog Training & Consulting (Formally LTC Training Center), a full service training center specializing in live seminars, video-format certification training programs, plain language technical manuals and educational support on CCTV. He is an active member of ASIS, ALAS, CANASA, NBFAA, NAAA and SIA. He is the recipient of numerous security industry awards, and is a regular contributor to Security Technology & Design magazine. Look for his columns to also appear regularly via SecurityInfoWatch.com and this website's e-newsletters. He can be contacted via email at [email protected].